如何对在操作中具有私有方法的ASP.Net MVC控制器进行单元测试

在尝试测试ASP.Net MVC控制器/操作时，我很难理解单元测试模式。

使用以下代码，我试图为ShowPerson()方法编写一个测试：

public class PersonController : Controller
{
    private IDataAccessBlock _dab;
    public PersonController()
        : this(new DataAccessBlock())
    { }
    public PersonController(IDataAccessBlock dab)
    {
        _dab = dab;
    }
    public ActionResult ShowPerson(PersonRequestViewModel personRequest)
    {
        var person = GetPersonViewModel(personRequest);
        return View("Person", person);
    }
    private PersonViewModel GetPersonViewModel(PersonRequestViewModel personRequest)
    {
        var personService = new CommonDomainService.PersonService(_dab);
        var dt = personService.GetPersonInfo(personRequest.Id);
        var person = new PersonViewModel();
        if (dt.Rows.Count == 1)
        {
            person.FirstName = dt.Rows[0]r["FIRSTNAME"]);
            person.LastName = dt.Rows[0]["LASTNAME"];
        }
        return person;
    }
}

我正在使用的测试（使用UnUnit和Moq）：

[Test]
public void ShowPerson_Action_Should_Return_Person_View()
{
    // Arrange
    string expected = "Person";
    Mock<PersonRequestViewModel> personRequestViewModelMock = new Mock<PersonRequestViewModel>();
    personRequestViewModelMock.SetupProperty(f => f.Id, 123456);
    Mock<IDataAccessBlock> mockDab = new Mock<IDataAccessBlock>();
    PersonController personController = new PersonController(mockDab.Object);
    // Act
    ViewResult result = personController.ShowPerson(personRequestViewModelMock.Object) as ViewResult;
    // Assert
    personRequestViewModelMock.Verify();
    result.Should().Not.Be.Null();
    if (result != null) Assert.AreEqual(expected, result.ViewName, "Unexpected view name");
}

一切似乎都很顺利，直到遇到线路if (dt.Rows.Count == 1)。我得到一个"Object reference not set to a instance of a Object"

我认为以下两行的书写方式一定有些怪异：

var personService = new CommonDomainService.PersonService(_dab);
var dt = personService.GetPersonInfo(personRequest.Id);

但我不知道从这里到哪里去。我有很多这样的代码。我是做错了什么，还是有实际的方法来测试这一点？

谢谢你的帮助或指点。